Gd3+-Doped α-CsPbI3 Nanocrystals with Better Phase Stability and Optical Properties

Black α-CsPbI3 perovskites are unable to maintain their phase stability under room conditions; hence, the α-CsPbI3 phase transforms into a thermodynamically stable yellow δ-CsPbI3 phase within a few days, which has a nonperovskite structure and high band gap for optoelectronic applications. This phase transformation should be prevented or at least retarded to make use of superior properties of α-CsPbI3 in optoelectronic applications. In this study, Gd3+ doping was employed with the aim of increasing the stability of α-CsPbI3. All doped α-CsPbI3 nanocrystals with various levels of Gd3+, between 5 and 15 mol %, have shown greater phase stability than that of the pure α-CsPbI3 phase from 5 days up to 11 days under ambient conditions. This prolonged phase stability can be attributed to three potential reasons: increased tolerance factor of the perovskite structure, distorted cubic symmetry, and decreased defect density in nanocrystals. Urbach energy values suggest the reduction of defect density in the doped nanocrystals. Also, use of 10 mol % Gd3+ as a dopant material increases the photoluminescence quantum yield from 70 to 80% and fluorescence lifetime of α-CsPbI3 from 47.4 to 64.4 ns. Further, density functional theory calculations are in a good agreement with the experimental results.